The present invention relates to frequency modulation in general, and more particularly to a method of and arrangement for frequency-modulating telecommunication carriers with baseband signals.
As well known in the art, baseband signals which contain the information to be transmitted and which will hereafter be referred to as message signals are modulated onto a carrier in order to facilitate or enable transmission over considerable distances. Various modulation techniques are known including frequency modulation with which the present invention is concerned.
There are two basic ways to accomplish frequency modulation at a desired carrier frequency. One is to modulate the local or reference oscillator in a synthesizer loop by the baseband message signal. A problem with this approach is the inability to maintain a linear relationship between the baseband signal amplitude and the carrier deviation when the carrier frequencies change. The other known approach is to modulate an oscillator at a non-zero intermediate frequency which is then translated to the desired carrier frequency by a third frequency. Even this approach has certain drawbacks. So, for instance, additional complicated control circuitry is required for the selection of a proper third frequency, and additional hardware must be provided, this hardware operating at radio frequency, resulting in interference problems.